Rotating machines, such as generators and motors, feature bearing assemblies (usually two) that support a shaft. The bearing assemblies maintain concentricity between a rotor assembly mounted on the shaft and a stator or housing assembly.
In electrical generators, such as those used to power the electrical systems of aircraft, the generator is connected to the main engine by a drive shaft. The rotor assembly and shaft rotate within the stator assembly. The shaft of most air cooled generators is supported for rotation in the housing using sealed bearing assemblies comprising grease lubricated ball bearings. In the course of normal generator operation, the bearings are subject to wear. Worn bearings, if not replaced, can eventually lead to bearing failure which will cause rotor rub and, because of the generator's kinetic energy, consequent severe damage. Hence, most rotating machines employed in sensitive applications follow a strict bearing replacement schedule with intervals short enough to fall outside the distribution curve of bearing wear-out failures.
In the aircraft industry, periodic scheduled maintenance is conducted based upon the operating time of the equipment. This is commonly referred to as "time-between-overhaul" (TBO). As part of a TBO maintenance program, a generator's bearings are replaced at predetermined service intervals.
The bearing replacement intervals are determined based upon worst-case scenarios of bearing wear, and the bearings are scheduled for replacement before any reasonable possibility of bearing failure can occur. As a result, the bearings are usually replaced long before they are actually worn out, and in many situations, the generators are disassembled and serviced unnecessarily before there is any immediate need for bearing replacement. This results in unnecessary servicing, and lost revenue associated with the equipment downtime.
Prior art has incorporated an auxiliary bearing system to keep the rotor supported in the event of a main bearing failure. This system features a smaller auxiliary bearing assembly mounted in close proximity, near each main bearing assembly. Under normal conditions, the main bearing assemblies support the shaft, while the auxiliary bearing assemblies idle or, in other words, spin freely along with the rotating shaft. This condition is made possible by an annular clearance such as, for example, 0.002 inch between the outer race of the auxiliary bearing and the generator housing; sometimes referred to as the bearing support wall.
In the absence of friction, no relative motion can exist between inner and outer races of the auxiliary bearing and no wear is incurred. In this manner, the auxiliary bearing assemblies remain ready to take over support of the shaft from the main bearing assembly, as soon as the main bearings fail or wear sufficiently for the internal clearance, here defined as the distance between the inner and outer races, to equal or exceed the annular clearance between the auxiliary bearing outer race and the bearing support wall.
Therefore, as a main bearing assembly begins to wear, the distance between its inner and outer race decreases. Since the main bearing outer race is frictionally engaging the bearing support wall, the shaft will slowly move closer to the bearing support wall as the main bearing wears. The annular clearance between the bearing support wall and the outer race of the auxiliary bearing will decrease as the main bearing wears until eventually the outer race of the auxiliary bearing will frictionally engage the bearing support wall. It is at this point where the auxiliary bearing begins to support the shaft.
In the prior art, the bearing support wall is metallic. However, because of the heavy weight or density of steel, the bearing support wall is usually constructed of aluminum with a thin layer of steel, commonly termed a steel liner on the surface which faces the generator shaft.
Completing the prior art system is a bearing failure sensor. This device announces the failure of the main bearing and is essential to achieve "on-condition" operation. It functions by alerting the operator or pilot that a main bearing assembly has failed and that the auxiliary bearing assembly has assumed support of the generator shaft. A typical prior art sensor is illustrated in FIG. 7 and has a grooved channel 90 into which is inserted a wire 92 partially covered with insulation 94. This prior art, in theory, provides adequate operation time to land the aircraft and replace the main bearings before the auxiliary bearings fail.
Prior art has dealt with bearing failure sensors in a variety of ways, though most have been labor intensive and difficult to implement.
U.S. Pat. No. 5,602,437 issued to Shahamat et al. attempted to address the concerns regarding unnecessary servicing. Shahamat discloses a bearing failure detector in the form of a disc positioned about the generator shaft. The disc itself includes an electrically conductive ring encased in an insulation layer but having the conductive ring exposed along the inside edge of the disc which faces the shaft. Initially, there is a set clearance between the shaft and the conductive ring. The ring is connected to a large resistance which is monitored by a control circuit.
Shahamat et al. teaches that as the bearings wear, the generator shaft will eventually contact the conductive ring, establishing a circuit where the electrical resistance monitored by the control circuit will approach zero and thereafter send an appropriate warning signal to the cockpit.
In Shahamat et al., the conductive ring surface which faces the shaft is bare. This configuration could provide false indications if airborne contaminants or moisture are present. Further, Shahamat et al. relies on the conductive ring contacting the generator shaft to indicate impending failure. This warning indication would only be for a limited time because the possibility exists that the rotating shaft wears through the conductive ring if the diameter of the ring is small.
Even though TBO results in unnecessary servicing of aircraft generators, TBO procedures are necessary because there has been no reliable means by which one could determine whether or not a generator's bearings were close to actual failure.